The present invention relates generally to electrical ionizers that produce stable charge-carrier production in gases with varying concentrations of electron attaching components. More particularly, the invention relates to ionizers suited for production test environments of semiconductor devices and component handlers and other environments that might be rendered inert by nitrogen and noble gases. In these environments, the mobility of gaseous charge carriers changes with gas composition and temperature.
Nitrogen gas is used to inert processes in many industries, and can purge areas cooled by the evaporation of liquid nitrogen. In recent years, static eliminators using nuclear (radioisotope), ultraviolet, soft x-ray, and corona discharge ionizers have been explored for use in nitrogen environments. The ionizers in static elimination equipment produce positive and negative charges that have the mobility needed to be drawn to static (stationary or fixed), unbalanced electric charges on surfaces. Controlled production of these gas-borne charge carriers is critical to static elimination.
The characteristics of electrical corona in high-purity nitrogen gas have been known for many years. To gain fundamental data on these corona, effort must be made to purify the nitrogen gas and to obtain cleanliness of the chamber, especially against in-leakage and outgassing. The negative charge carriers formed in the discharge are free electrons and these do not readily attach to atomic or molecular nitrogen species. As a result, the mobility of the negative charge carriers is about 1000 times that for the positive charge carriers. In industrial applications the impurity level is less controllable and often unknown. Then, free electrons will attach to oxygen and other electronegative impurities. Negative carrier mobility, which influences factors that determine performance of an ionizer such as ion currents, sparkover conditions, and migration of charge, can change significantly with the composition of the environment. The carrier mobility also depends upon temperature. To serve the static elimination goals of industrial applications successfully, the ionizer must be sufficiently controllable over the variability in ion mobility.
Each of the alternative technologies (nuclear, UV, x-ray) produces positive ion and free electron pairs in nitrogen. The balance of these ionizers, however, is not easily controlled in air, let alone dilute air-nitrogen gas mixtures and over the temperature range of interest herexe2x80x94213 to 433 K. Also, the alternative ionizers can introduce hazards to the work place. An electrical ionizer is, therefore, preferred if its balance can be controlled.
The injection of electron attaching species in the vicinity of the corona discharge was proposed recently towards stabilization of the corona. This method can serve applications that can tolerate gas injection and the added cost of a gas handling system. Success of this method diminishes at low temperatures where there is an additional burden of a high level of drying and filtration of the injected gas. The present invention eliminates the need for an injection-gas handling system and is applicable to broader temperature and conditions of gas contamination.
The environmental chamber, used to test the invention, is a commercial semiconductor device handler and tester. This chamber is heated by electrical methods and cooled by the evaporation of liquid nitrogen. Excellent control of the temperature is achieved (less than xc2x11 K), and gas flows are maintained in the equipment both to gain temperature uniformity and to convey charge carriers. Apart from the challenges of the thermal environment, nitrogen poses very different conditions for static elimination than those for operation in air. Also, the mechanical complexity of the equipment sets additional challenges for balanced static elimination. Although the test environment is complex, the semiconductor device handler offers a flexible environment for the development of static eliminators.
It is, therefore, the purpose of this invention to provide an ionizer that can be balanced under conditions of variable ion mobility and complex mechanical constraints. Although tested in the environment of a device handler, the resulting ionizer is appropriate for use in other industrial applications with similar requirements.
The present invention provides an ionizer that creates a corona current distribution having a balanced flow of positive and negative ions in a variable ion mobility gaseous environment. The balanced flow of positive and negative ions is directed toward a workspace or target located in the gaseous environment downstream from the ionizer. The ionizer comprises a counterelectrode, a positive ion emitter, a negative ion emitter, and a control circuit. The counterelectrode serves the counterelectrode function for the corona emitters. It isolates the emitter-current distributions, so that current is prevented from passing from the negative polarity emitter to the positive-polarity emitter and vice versa. Although complete isolation is not possible the current is so limited as to allow each emitter-counterelectrode set to operate independently. The positive ion emitter directs positive ions towards only a first ion-collecting surface of the counterelectrode. The negative ion emitter directs negative ions towards only a second ion-collecting surface of the counterelectrode. The positive and negative ion emitters are spatially isolated from each other so that the outputs of each of the emitters do not reach the other emitter. The control circuit controls the output of at least one of the positive and negative emitters so as to cause a balanced flow of positive and negative ions to be emitted from the ionizer and directed towards the workspace or target. This creates a static-free environment at the workspace or target.
The control circuit comprises a positive voltage controlled power supply, a negative fixed voltage potential current limiting power supply, and a balance sensor. The positive voltage controlled power supply controls the positive ion emitter. The negative fixed voltage controlled potential current limiting power supply controls the negative ion emitter. The balance sensor is located near the workspace or target. The output of the balance sensor is used to control the positive power supply. Proper control of the positive power supply by the balance sensor maintains a balanced ion state near the workspace or target. The control circuit may increase the voltage from the positive voltage controlled power supply for control of the positive ion emitter when environmental temperature decreases.
The ionizer emitters may be configured to have point-to-plane geometry. In the point-to-plane geometry, the counterelectrode is a single plane having two opposing ion collecting surfaces, and the positive and negative ion emitters are needle electrodes. The needle-to-tube ionizer emitter construction comprises a counterelectrode, and positive and negative emitters. The counterelectrode includes a first tube and a second tube. The positive and negative ion emitters are needle electrodes. The positive needle electrode is disposed in the first tube wherein the positive ions are directed towards the inside surface of the first tube. The negative needle electrode is disposed in the second tube wherein the negative ions are directed towards the inside surface of the second tube. The first and second tubes may be cylindrically shaped stainless steel tubes having a Fiberglass G-7 insulation barrier.
The positive ion emitter and the negative ion emitter each have a tip that may be directed downstream from the ionizer.
In the point-to-plane arrangement, the positive ion emitter and negative ion emitter may each include a supporting tube that supports the emitters and allows rotational positional adjustment. The rotational positional adjustment of the emitters allows for the independent operation and forward projection of the balanced flow of positive and negative ions towards the workspace or target. Additionally, the supporting tube may be used as an air plenum for gas-injection into the environment.
Another embodiment of the present invention provides a method of creating a corona current distribution having a balanced flow of positive and negative ions which are directed toward a workspace or target. In the method, a variable ion mobility gaseous environment is provided. The workspace or target is located in the gaseous environment. An ionizer is operated in the gaseous environment to create the corona current distribution. The workspace or target is located downstream from the ionizer. The ionizer includes a positive ion emitter and a negative ion emitter. The negative ion emitter is controlled with a negative fixed voltage potential current limiting power supply. The positive ion emitter is controlled with a positive voltage controlled power supply based on the output signal of a balance sensor located near the workspace or target so as to cause a balanced flow of positive and negative ions to be emitted from the ionizer and directed towards the workspace or target. This process creates a static-free environment at the workspace or target.
Another embodiment of the present invention provides a method of creating a corona current distribution having a balanced flow of positive and negative ions which are directed toward a workspace or target. In the method, a variable ion mobility gaseous environment is provided. The workspace or target is located in the gaseous environment. An ionizer is operated in the gaseous environment to create the corona current distribution. The workspace or target is located downstream from the ionizer. The ionizer includes a counterelectrode, a positive ion emitter, a negative ion emitter, and a control circuit. Positive ions are directed from the positive ion emitter towards only a first ion collecting surface of the counterelectrode. Negative ions are directed from the negative ion emitter towards only a second ion collecting surface of the counterelectrode. The positive and negative ion emitters are spatially isolated from each other so that outputs of each of the emitters do not reach the other emitter. The control circuit controls the output of at least one of the positive and negative emitters so as to cause a balanced flow of positive and negative ions to be emitted from the ionizer and directed towards the workspace or target, thereby creating a static-free environment at the workspace or target.